Where should I place the probe in my snake's viv?

I purchased a digital thermometer for my ball python's vivarium. I am uncertain as to where to place the probe to assure the most accurate reading. The thermometer came with no suggestions. Do I place the probe in the back center or in the hottest area of the viv? Please advise and I do thank you.|||I use 2 digital thermometers placed on each end. The probes are both placed on the surface of the substrate.|||If I were you I would place it in the coldest spot in the cage because then you will now the lowest temperature instead of the highest. That way if it is like 80 you will now that that is the lowest temperature and the hotter side has to be warmer.|||Ideally, you should know what the hottest and the coldest temps are. In my BP's cage though I have only one digital thermometer at the hottest part of the cage to make sure the thermostat I have for his heat source is working right. Once you have the cage set up with the temp range you want the cold temp will generally remain the same as long as the hot side temp and the temp of the room stay the same. I use a digital infrared thermometer to check the low temp every couple days and it doesn't really change much - only about a degree. I really like the infrared thermometer. You can check air or surface temp anywhere you want in the cage. The simple ones look like small laser pointers and are only about $20-$25.